This invention relates generally to an improved system and method for anticipating and controlling overspeed in a combined cycle turbine of the type having a gas turbine and steam turbine on a single shaft. More particularly, the invention relates to an improved system and method for limiting overspeed during transient load conditions in a combined cycle turbine driving a generator which is synchronized to an electrical load grid, including guarding against overspeed in event of electrical load loss. Such a combined cycle turbine and a method for starting up and synchronizing it with a unified control system was disclosed pending application in Ser. No. 431,892, filed Nov. 6, 1989 and assigned to the present assignee.
In some large combined cycle power plants the steam turbine and gas turbine are solidly coupled on a single shaft to drive a single electrical generator. The primary source of energy input to the rotating machine is the fuel which is burned in the gas turbine combustors. This shows up almost immediately as power delivered by the gas turbine. The waste heat from the gas turbine generates steam. This steam, which is generated by a heat recovery steam generator (HRSG), is utilized by a steam turbine as a secondary source of power input to the rotating shaft train. While there is some time lag before heat from the gas turbine exhaust gas manifests itself as a power input source in the form of steam available at the turbine control valves, the control of the two sources of energy must be coordinated in order to properly control and protect the rotating machinery.
When synchronized with the electrical grid the speed of the machine is determined by the frequency of the grid. Of the total mechanical power produced from the fuel to drive the generator, approximately two-thirds is produced by the gas turbine and one-third by the steam turbine from the thermal energy recovered from the gas turbine exhaust. In most cases, all of the steam produced by the heat of the gas turbine exhaust is expanded through the steam turbine. In other cases, some of the steam is extracted from the power cycle for process uses. If all of the steam produced by the gas turbine exhaust is expanded through the steam turbine, and the unit is synchronized, the steady state control of electrical output, is achieved entirely by controlling gas turbine fuel flow, with the steam control valve or valves maintained in the fully open position. When not synchronized, on the other hand, either fuel flow to the gas turbine, steam flow to the steam turbine, or both, must be controlled to control speed, and there is not always a direct relationship between the two.
Gas turbines and steam turbines control speed (or load) by increasing and decreasing fuel flow and steam flow, respectively, in response to an error signal generated in the control system. An error signal is the difference between a reference (desired value) of an operating condition and the actual measured value of the operating condition. The gas turbine control system utilizes several such error signals to develop several fuel command signals which are applied to a "minimum value gate". The smallest fuel command signal generated by the startup fuel schedule is selected by the minimum value gate unless temperature or other limitations have a smaller fuel command signal. As speed approaches the governor set point, the speed error requires the smallest fuel command signal and becomes the controlling signal. An integrated gas turbine control system providing for open loop programmed start-up control with a number of closed loop constraints simultaneously controlling the gas turbine in accordance with operating conditions such as temperature, speed and acceleration is described in U.S. Pat. No. 3,520,133 issued Jul. 14, 1970 to Daniel Johnson and Arne Loft. Once the unit is at rated speed and synchronized, load is controlled by adjusting the fuel flow in accordance with the setting of the governor load set point.
A steam turbine is self-starting as soon as steam is admitted through the control valve, but due to need to allow temperatures to equalize in the rotor and shell, startup programs have been developed for starting and loading a steam turbine. Combining acceleration and speed control through the use of a "minimum value gate" are shown in U.S. Pat. No. 3,340,883--Peternel, issued Sep. 12, 1967. Once a steam turbine is synchronized, the load is controlled by adjusting steam flow through the control valve in accordance with the setting of a load set point, as shown in U.S. Pat. No. 3,097,488 issued to M. A. Eggenberger et al on Jul. 16, 1963.
Unified control systems have been proposed for single shaft combined cycle plants with supplemental firing of fuel in the heat recovery steam generator which attempted to force a programmed load split between the gas turbine and the steam turbine, such a system being disclosed in U.S. Pat. No. 3,505,811 to F. A. Underwood issued Apr. 14, 1970. However, improved thermodynamic performance can be achieved by designing the system so that the steam valve remains in the fully open position. In this way, the steam turbine accepts the total generation capacity of the steam generator over the entire load range without responding to small or slow speed variations which would require steam valve adjustment.
As load is increased on the gas turbine, more heat energy will flow with the exhaust gas to the HRSG where it will cause an increase in steam flow to the steam turbine. This will cause the steam pressure to rise so that the steam turbine will absorb this flow without any control action. A reduction in gas turbine load will, in similar manner, result in a reduced steam flow to the steam turbine. Thus, the steam turbine will follow the load changes on the gas turbine with some time delay. Hence, normal control of a combined cycle plant on a single shaft under slowly varying load conditions is by means of increasing or decreasing rate of fuel flow with change in load.
While this provides optimum thermodynamic performance under steady state or slowly varying load changes, disturbances in steady or quasi-steady operation may occur. It would be desirable to provide for proportional control of both fuel flow and steam flow above rated speed. Although, a gradual rise in shaft speed above rated speed will cause the gas turbine speed control to reduce fuel flow and hence power to the shaft in a proportional manner with speed rise, this may not be adequate during transient load change. It would be desirable to have a system in which, as long as the shaft speed was below a preset value, the steam turbine would only respond by a reduced output as the steam flow from the HRSG is reduced, but in which a rise in combined shaft speed above the preset value would cause the steam valves to go closed in a manner proportional to the speed rise. This would reduce the steam flow to minimum flow level and hence shut off the steam flow as a contributor to excessive overspeed.
Under more severe, transient conditions, such as in the event of sudden loss of full electrical load, the above described proportional action of both fuel flow and steam flow may not occur fast enough to limit the speed rise of the unit to a value that will not cause the overspeed trip to activate, typically at 110% rated speed. Modern fossil fired steam turbines use a power-load unbalance system to control overspeed to a value below that of the setting of the overspeed trip. This permits the unit to experience a load rejection, yet remain running under speed control at or near synchronous speed. Thus, the unit can, if desired, continue to carry station auxiliary load and also be in a condition for prompt resynchronizing with the system. Such power load unbalance systems are shown in U.S. Pat. No. 3,198,954 in the name of M. A. Eggenberger et al issued Aug. 3, 1965 or in U.S. Pat. No. 3,601,617 to DeMello et al issued Aug. 24, 1971.
The prior art power-load unbalance systems in steam turbine generators only provide for one power input. The anticipation of overspeed is more complex and difficult in a combined cycle having both steam turbine and gas turbine on a single shaft.
Accordingly, one object of the present invention is to provide an improved method for controlling and preventing overspeed in a single shaft combined cycle turbine during transient load disturbances.
Another object of the invention is to provide an improved power load unbalance control system for anticipating and preventing overspeed in a combined cycle turbine.
Another object of the invention is to provide an improved unified control system for anticipating and preventing overspeed in a single shaft combined cycle plant, including proportional control between steam turbine and gas turbine during transient load conditions.